1. Field of the Invention
This invention relates to a paperboard container in which a sealing tape is applied to the overlying heat sealing ends of a paperboard blank in order to reinforce the seal of the overlying ends.
2. Prior Art
Various paperboard containers having polyethylene coatings on both surfaces are used in a wide variety of industrial uses including containers for milk, juice, soysource, sake, etc. Depending upon the construction, there are two types of containers. One type is constructed by cutting a blank paperboard container out of a flat paperboard blank. The other type is formed by overlaying both sides of a paperboard blank transversely, so that the blank is formed into a tube with bottom panel extensions.
The above described containers, however, have a defect in that when a cross section of the overlying paperboard blank inside of the container comes into contact with the contents of the container (a liquid), the contents penetrate the paperboard wall of the base blank. Thus, the strength of the paperboard blank deteriorates causing the container to become deformed and its strength is greatly decreased.
In order to solve this problem, Japanese Utility Model Laid-Open Application No. 51-71232 disclosed a paperboard container wherein both ends of the container are overlapped to form a gable-top and are brought down to one side. Then, the ends are heat sealed to the outer wall. Japanese Utility Model Laid-Open Application No. 51-49318 disclosed a paperboard container in which the polyethylene coating inside the container is spread at the end of a paperboard blank so that it covers a cross section of the end of the paperboard. Then, it is heat sealed to the outer polyethlene coating.
A method for heat sealing the inside of the overlying end parts of a container has also been proposed wherein a polyethylene strip is used as a sealing tape. The improvement provided in this method, as disclosed in Japanese Patent Laid-Open Application No. 50-90487, lies in the use of a three-layer tape consisting of a material with a higher softening point which uses polyethylene on the front and back surfaces of this material. Among the well known conventional art, the paperboard container described in Japanese Utility Model Laid-Open Application No. 51-71232 has an unsightly appearance due to the extremely thick overlying part and the paperboard container described in Japanese Utility Model Laid-Open Application No. 51-49318 has inferior production efficiency and is not suitable for mass production since the conventional method of cutting a container blank from a flat paperboard blank cannot be applied.
In conventonal containers a sealing tape is used as described in the following paragraph.
Referring to FIG. 5, one side of a sealing tape 20 is heat sealed to one web end 41A. Then the web with the sealing tape is immersed into a disinfectant and dried. Next, both web ends 41A and 41B are overlapped. Lastly, the other side of the sealing tape 20 is heat sealed to the other web 41B. The tape 20 is made of polyethylene and can be damaged easily due the heat of the steam which is generated due to the evaporation of water at the cross of the edge of the web. In order to solve this problem, Japanese Patent Laid-Open Application No. 56-14544 proposed a paperboard container witha tape 30 consisting of three layers, as shown in FIG. 6. However, there are two problems with this prior art. One problem lies in that both of the tape edges consist of three laminated layers and they are in contact with the contents of the container (a liquid). Furthermore, since an adhesive agent (called an "anchor") is used, between the polyethylene layer 32 and the base layer 33, the contents of the container (a liquid) are brought into contact with an adhesive agent.
If the base layer 33 is made of aluminum foil, it is likely that the aluminum will dissolve if it is used to store a strongly acidic juice for a long period of time. Another problem lies in that steam is generated when both end parts 41A and 41B of the paperboard blank are heat sealed. Generally, when a paperboard container is constructed using a tubular form, the end parts 41A and 41B of the paperboard web 41, having polyethylene coatings 42A, 42B, 42C and 42D on both the front and back surfaces, are overlapped, heat sealed and formed into a tubular shape. During the heat sealing process the steam is released through a triangle part 35 from the cross section of the heated end part 41A of the paperboard blank. In this method, the polyethylene coating 32 of the sealing tape 30 and the polyethylene layer 42A of the paper-board blank are wholly heat sealed so that the steam released from the end part 41A cannot be removed. Thus, steam is often entrapped in the form of specks and serves to destabilize the function of the sealing tape 30.
When the specks spread, the seals deteriorate remarkably.